This invention relates to an improved method of making an electrode assembly, such as a color-selection structure for a CRT (cathode-ray tube) and to a color-selection structure made according to the method.
A conventional shadow-mask-type color television picture tube, which is a CRT, includes generally an evacuated envelope having therein a target comprising an array of phosphor elements of three different emission colors arranged in color groups in cyclic order, means for producing three convergent electron beams directed toward the target, and a color selection structure having an apertured masking plate closely spaced from the target. The masking plate shadows the target, and the differences in convergence angles permit the transmitted portions of each beam to select and excite phosphor elements of the desired emission color.
At about the center of the color-selection structure, the masking plate of a commercial CRT intercepts all but about 18% of the beam currents; that is, the plate is said to have a transmission of about 18%. Thus, the area of the apertures of the plate is about 18% of the area of the masking plate. Since there are no focusing fields present, a corresponding portion of the target is excited by the beamlets of each electron beam.
Several methods have been suggested for increasing the transmission of the masking plate; that is, increasing the area of the apertures relative to the area of the plate, without substantially increasing the excited portions of the target area. In one approach, each of the apertures of the color-selection structure is defined by a quadrupolar electrostatic lens which focuses the beamlets passing through the lens in one transverse direction and defocuses them in the orthogonal transverse direction on the target depending upon the relative magnitudes and polarities of the electrostatic fields comprising the lens.
In one type of quadrupolar-lens color-selection structure described in U.S. Pat. No. 4,059,781 to W. M. van Alphen et al,., a strong focusing quadrupolar lens is generated from voltages applied between two sets of substantially-parallel conducting strips, each set being orthogonally positioned with respect to the other, and insulatingly bonded at the intersections of the strips.
In other type of quadrupolar-lens color-selection structure described in the same patent, interwoven or overlaid horizontal and vertical conductors are used. The conductors comprise conductive wires or strips which are covered with glass insulation. The conductors are interwoven or pressed against each other in the desired manner in a mold and heated so that the conductors with the glass insulation fuse together without making electrical contact with each other. The glass insulation must be removed from the conductors, with the exception of the crossings, to avoid undesired charging of the glass when the structure intercepts the electron beams. This structure has the disadvantages that complete removal of the glass insulation is difficult, and when incomplete, adversely affects the operation of the color-selection structure. Furthermore, removal of the glass insulation at the crossings must be avoided, since this renders the structure inoperative and is irreversible, since insulation cannot be reapplied.
In a second approach to increasing the transmission of the masking plate, each aperture in the masking plate is enlarged and split into two adjacent windows by a conductor. The two beamlets passing through the windows of each aperture are deflected around the conductor towards one another, and both beamlets fall on substantially the same are of the target. In this second approach, the transmitted portions of the beams are also focused in one transverse direction and defocused in the orthogonal transverse direction.
One effort at such a combined deflection-and-focus color-selection means is described in West German Offenlegungschrift No. 2,814,391, published Oct. 19, 1978. That publication discloses a CRT having a target, as normally viewed, comprised of a mosaic of vertical phosphor stripes of thre different emission colors arranged cyclically in triads (groups of three different stripes), means for producing three convergent horizontally in-line electron beams directed towards the target, and a color-selection structure located adjacent the target. The color-selection structure compirses a metal-masking plate having therein an array of substantially square apertures arranged in vertical columns, and an array of narrow vertical conductors insulatingly spaced from the masking plate, with each conductor substantially centered over the apertures of each of the columns of apertures. Each apertures is also centered over a triad of phosphor stripes. Viewed from the electron-beam-producing means, the conductors divide each aperture into two essentially-equal horizontally-coadjacent windows. This prior color-selection structure has windows with a width-to-height ratio of about 0.46 and transmits about 44% or less of the electron beams.
This latter device constructed by the insulated-conductor method described in the aforementioned van Alphen et al. patent would have the same disadvantages enumerated with respect to those structures.
In any of these and similar structures, it is a major problem to construct an array of conductors that are electrically insulated from one another with the insulation not exposed to the electron beams as is required for use in a CRT color-selection structure. A need therefore exists for a focusing structure which can be fabricated inexpensively, reproducibly and reliably.